This invention relates generally to turbine components and more particularly to a generally cylindrical connector segment that connects a combustor liner to a transition piece in land based gas turbines.
Traditional gas turbine combustors use diffusion (i.e., non-premixed) flames in which fuel and air enter the combustion chamber separately. The process of mixing and burning produces flame temperatures exceeding 3900 degrees F. Since conventional combustors and/or transition pieces having liners are generally capable of withstanding for about ten thousand hours (10,000) a maximum temperature on the order of only about 1500 degrees F., steps to protect the combustor and/or transition piece, as well as the intervening connecting segment, must be taken. This has typically been done by film-cooling which involves introducing the relatively cool compressor air into a plenum surrounding the outside of the combustor. In this prior arrangement, the air from the plenum passes through louvers in the combustor liner and then passes as a film over the inner surface of the combustor liner, thereby maintaining combustor liner integrity.
Because diatomic nitrogen rapidly disassociates at temperatures exceeding about 3000xc2x0 F. (about 1650xc2x0 C.), the high temperatures of diffusion combustion result in relatively large NOx emissions. One approach to reducing NOx emissions has been premix the maximum possible amount of compressor air with fuel. The resulting lean premixed combustion produces cooler flame temperatures and thus lower NOx emissions. Although lean premixed combustion is cooler than diffusion combustion, the flame temperature is still too hot for prior conventional combustor components to withstand.
Furthermore, because the advanced combustors premix the maximum possible amount of air with the fuel for NOx reduction, little or no cooling air is available, making film-cooling of the combustor liner and transition piece impossible. Thus, means such as thermal barrier coatings in conjunction with xe2x80x9cbacksidexe2x80x9d cooling have been considered to protect the combustor liner and transition piece from destruction by such high heat. Backside cooling involved passing the compressor air over the outer surface of the combustor liner and transition piece prior to premixing the air with the fuel.
Lean premixed combustion reduces NOx emissions by producing lower flame temperatures. However, the lower temperatures, particularly along the inner surface or wall of the combustor liner, tend to quench oxidation of carbon monoxide and unburned hydrocarbons and lead to unacceptable emissions of these species. To oxidize carbon monoxide and unburned hydrocarbons, a liner would require a thermal barrier coating of extreme thickness (50-100 mils) so that the surface temperature could be high enough to ensure complete burnout of carbon monoxide and unburned hydrocarbons. This would be approximately 1800-2000 degrees F. bond coat temperature and approximately 2200 degrees F. TBC (Thermal Barrier Coating) temperature for combustors of typical lengths and flow conditions. However, such thermal barrier coating thicknesses and temperatures for typical gas turbine component lifetimes are beyond current materials known capabilities. Known thermal barrier coatings degrade in unacceptably short times at these temperatures and such thick coatings are susceptible to spallation.
Advanced cooling concepts now under development require the fabrication of complicated cooling channels in thin-walled structures. The more complex these structures are, the more difficult they are to make using conventional techniques, such as casting. Because these structures have complexity and wall dimensions that may be beyond the castability range of advanced superalloys, and which may exceed the capabilities of the fragile ceramic cores used in casting, both in terms of breakage and distortion, new methods of fabricating must be developed to overcome these prior limitations. Possible geometries for enhanced cooling are disclosed in, for example, commonly owned U.S. Pat. Nos. 5,933,699; 5,822,853; and 5,724,816. By way of further example, enhanced cooling in a combustor liner is achieved by providing concave dimples on the cold side of the combustor liner as described in U.S. Pat. No. 6,098,397.
In some gas turbine combustor designs, a generally cylindrical segment that connects the combustion liner to the transition piece and also requires cooling. This so-called combustor liner segment is a double-wall piece with cooling channels formed therein that are arranged longitudinally in a circumferentially spaced array, with introduction of cooling air from one end only of the segment. The forming of these cooling channels (as in U.S. Pat. No. 5,933,699, for example) has been found, however, to produce undesirably rough surfaces, and in addition, the design does not allow for the spaced introduction of coolant along the segment.
Accordingly, there is a need for enhanced cooling in the segment connecting the combustion liner and transition piece that can withstand high combustion temperatures.
This invention provides a generally cylindrical double-wall segment for connecting the combustion liner and the transition piece with enhanced cooling achieved by the inclusion of concavity arrays on one or both major surfaces of each cooling channel, thereby providing as much as 100% cooling improvement. As a result, the channels may then also be extended by as much as two times their original length without increasing the volume of required cooling air. This arrangement also allows the cooling air to be fed in by impingement cooling holes spaced axially along the segment, rather than forced in only at one end of the segment.
In the exemplary embodiments, one or both major surfaces of the double-walled cooling channels are machined to include arrays of concavities that are generally closely spaced together, but may vary in spacing depending upon specific application needs. The spacing, cavity depth, cavity diameter, and channel height determine the resulting thermal enhancement obtained. The concavities themselves may be hemispherical, partially hemispherical, ovaloid, or non-axisymmetric shapes of generally spherical form. Cooling air is either introduced at one end of the channels, or alternately, through axially spaced impingement cooling holes, in combination with the cooling air inlet at one end of the segment.
The formation of arrays of surface concavities on the xe2x80x9chotxe2x80x9d side of the double-walled channels creates a heat transfer enhancement by so-called whirlwind effect from each cavity. The placement of similar arrays on the xe2x80x9ccoldxe2x80x9d surface also serves to enhance heat transfer if the walls are spaced closely together. Due to the bulk vortex mixing motion of the flow interaction with the cavities, the friction factor increase is small compared to that of a smooth surface. This overall cooling enhancement allows less total coolant to be used at any location in the channels. Moreover, by spacing the introduction of cooling air into the channels using impingement holes, the resultant effect is that the overall length of the enhanced double wall segment may be extended by about two times, without the use of additional coolant.
Accordingly, in its broader aspects, the present invention relates to a connector segment for connecting a combustor liner and a transition piece in a gas turbine, the connector segment having a substantially cylindrical shape and being of double-walled construction including inner and outer walls and a plurality of cooling channels extending axially along the segment, between the inner and outer walls, the cooling channels defined in part by radially inner and outer surfaces, wherein at least one of the radially inner and outer surfaces is formed with an array of concavities.
In another aspect, the invention relates to a connector segment for connecting a combustor liner and a transition piece in a gas turbine, the connector segment having a substantially cylindrical shape and being of double-walled construction including inner and outer walls and a plurality of cooling channels extending axially along the segment, between the inner and outer walls, the cooling channels defined in part by radially inner and outer surfaces; wherein both of the radially inner and outer surfaces are formed with an array of concavities; and further comprising axially spaced holes in the outer wall communicating said plurality of cooling channels.
The invention will now be described in detail in conjunction with the following drawings.